1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a unique control rod cluster in which the worth of the control rodlets varies axially in a manner which compensates for enthalpy rise across the core as power level changes.
2. Description of the Prior Art
In a typical nuclear reactor, such as a pressurized water type, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles.
One common arrangement utilizing control rods in association with a fuel assembly can be seen in U.S. Pat. No. 4,326,919 to Hill and assigned to the assignee of the present invention. This patent shows an array of control rods supported at their upper ends by a spider assembly, which in turn is connected to a control rod drive mechanism that vertically raises and lowers (referred to as a stepping action) the control rods into and out of the hollow guide thimbles of the fuel assembly. The typical construction of the control rod used in such an arrangement is in the form of an elongated metallic cladding tube having a neutron absorbing material disposed within the tube and with end plugs at opposite ends thereof for sealing the absorber material within the tube. Generally, the neutron absorbing material is in the form of a stack of closely packed ceramic or metallic pellets which only partially fill the tube, leaving a void space or axial gap between the top of the pellets and the upper end plug in defining a plenum chamber for receiving gases generated during the control operation. A coil spring is disposed within this plenum chamber and held in a state of compression between the upper end plug and the top pellet so as to maintain the stack of pellets in their closely packed arrangement during stepping of the control rod.
Thus, control rods affect reactivity by changing direct neutron absorption. Control rods are used for fast reactivity control. A chemical shim such as boric acid dissolved in the coolant is used to control long term reactivity changes. More uniformly distributed throughout the core, the boron solution leads to a more uniform power distribution and fuel depletion than do control rods. The concentration of boron is normally decreased with core burnup or time to compensate for fuel depletion and fission product buildup.
A pressurized water nuclear reactor (PWR) operating at full power with no control rods inserted into the fuel assemblies in its core will operate near zero axial offset. If this is not initially true, as burnup progresses, it will be. Also, during full power operation, the coolant density decreases with core height as heat is carried away from the fuel rods. However, if power is then reduced by increasing dissolved boron concentration or some other uniform poison change, the power will swing to the top of the core. The reason this happens is because, as power is reduced, the enthalpy rise across the core is reduced. (A rise in enthalpy refers to the rise in heat content of the water as its temperature rises). Thus, the coolant (or water) density change across the core is reduced which reduces the difference in reactivity between the bottom and top of the core. Relative to the bottom of the core, the top becomes more reactive since the water density is increasing there relative to the bottom. This causes power to shift to the top of the core.
One solution to this problem is to insert the control rods of the aforementioned conventional design, instead of changing the boron concentration, to change power level. Since the control rods enter the core from the top in the PWR, they will tend to drive the power to the bottom of the core and thus they will tend to compensate for the tendency of the power to swing to the top of the core at reduced power. While use of conventional control rods makes changes in the power level possible without large swings in axial offset, the increases in axial peaking factor during power changes demonstrates that use of this method is not problem-free.
Consequently, a need exists for improvements in enthalpy rise compensation, which will effectively resolve the problem of skewing reactivity toward the core top but which will not raise a host of new problems in the process.